EDM apparatuses utilizing a moving wire electrode are commonly presently used in making precise cuts and shaping various electrically conductive materials or workpieces. Typically, the wire electrode is wound around and travels between two turning spools and a pair of wire guides or guide assemblies wherethrough the wire travels are used to accurately situate or guide the moving wire electrode at the proper location and/or at angle with respect to the workpiece being cut. The wire and workpiece are placed at different electrical potentials and a controlled electrical spark traveling between the wire and workpiece causes the workpiece to be eroded and cut or shaped in a desired configuration.
It is electrically advantageous to place the electrical current on the moving wire electrode as close to the workpiece as possible and, thus, current pickups are generally placed within or in close proximity to the guide assemblies. An effective way of placing electrical current on the moving wire electrode is to cause the wire to slide or rub against an electrically charged surface, thus, transferring the electrical current therethrough to the moving wire electrode.
Various electrical feed devices have been developed whereat the moving wire electrode slidingly rubs against a conductive surface. In some of these devices, a current pickup is pushed towards the moving wire electrode or is placed in the normal straight line of wire travel and the tension in the wire causes the wire to rub against the current pickup thereby making the electrical contact. A typical current pickup of this character is disclosed in U.S. Pat. No. 4,539,459. Yet other current pickup assemblies, such as those disclosed in U.S. Pat. No. 4,559,433, utilize a plurality of current pickups that are pushed against the moving wire electrode or placed in the normal straight line of travel. In addition, as disclosed in Japanese Patent Application No. 59-195654 published Apr. 18, 1986, current pickups with a bore therethrough have been devised wherein the current pickup bore is offset from the straight line of wire travel and, thus, the moving wire electrode rubs against the interior surface of the bore and making the necessary rubbing electrical contact. So as to increase the life of this current pickup due to wear occurring on the bore wall, the contact pickup is indexed or rotated about the bore axis.
The prior current pickups and assemblies, however, have substantial shortcomings and drawbacks. At the outset, because the moving wire electrode rubs against the current pickup on one side thereof, for example, on one side of the current pickup bore surface, the wire electrode becomes slightly deformed or a "curl", also referred as memory, is placed on the wire thus causing inaccurate cutting and shaping of the workpiece. Without drastically pushing the wire electrode off of the normal straight line of travel with the current pickup, the tension in the wire is minimal and rubbing electrical contact with the current pickup is inefficient. With the current pickup being generally in the normal straight line of wire travel, manual and/or automatic threading of the moving wire electrode through the guide assembly and the Apr. pickup 1986, generally quite difficult. Further, although the present current pickups with a bore can be indexed or rotated, a groove often time forms in the bore wall and the current pickup must be rotated sufficiently so as to prevent the wire electrode from slipping back into that groove, thereby also wasting potential contact surface area therebetween.
Accordingly, a need exists for a current pickup that generally prevents inaccuracies due to curl, provides for increased tension in the wire and, therefore, also minimizing electrical resistance, provides for easy manual and automatic threading through the guide assembly and current pickup and, further, increases the useful life of the current pickup and utilizes a maximum amount of the current pickup bore surface.